Kaolin clay pigments used by the paper and paint industry are available in both uncalcined (hydrated) and calcined (dehydrated) grades. When preparing aqueous coating or paper filling compositions containing such pigments, it is frequently desirable to provide the clay in the form of a concentrated suspension (slurry) which is sufficiently fluid at both high and low rates of shear to be handled by conventional mixers and pumps.
When the clay is hydrated and has a limited content of particles larger than 2 microns (equivalent spherical diameter), it is relatively simple to produce a stable high solids (70%) suspension of the clay. A strong deflocculant (dispersant) such as tetrasodium pyrophosphate (TSPP) is added to a filter cake of the acid clay. The cake of hydrated clay is typically at about 60% solids. Additional dry clay is incorporated with mild agitation to such cake until the suspension has the desired high solids content. The TSPP is usually employed in amount within the range of 0.3% to 0.5% based on the dry clay weight. This corresponds to the use of 6 to 10 lbs. TSPP/ton of clay. The resulting suspension is stable in the sense that when the suspension is allowed to stand there is minimal settling of particles to form a dense sediment and minimal formation of a clear or cloudy supernatant liquid layer. This is attributable to the fact that suspensions of fine hydrated clay are fairly viscous and contain only small amounts of coarse particles. Few particles of clay, if any, have sufficient mass to settle under the influence of gravity.
In the case of clay pigments that contain significant amounts of coarse particles, especially particles larger than 2 microns, and which contain a low content of ultrafine particles, there is a marked tendency of coarse particles to settle out of deflocculated suspensions of the clay. For example, 70% solids deflocculated suspensions of relatively coarse filler grades of hydrated kaolin clay tend to form hard sediments during shipment or storage. These filler clays usually contain at least 20% by weight of particles larger than 5 microns and at least 35% larger than 2 microns.
A conventional method for maintaining various particulate solids in suspension in fluid media is to thicken the suspending media with suitable colloidal additives. This principle has been advocated to prevent sedimentation in high solids suspensions of filler grades of clay. In accordance with the teachings of U.S. Pat. No. 3,130,063 to Millman et al, an inorganic polymeric thickening agent, preferably CMC, is added to a previously deflocculated suspension of coarse filler clay in amount sufficient to thicken (and thereby stabilize) the suspension. However, organic polymers such as CMC are subject to bacterial degradation. Consequently, clay slurries stabilized with such polymers may arrive at their destination in the form of gray or black masses having a putrid odor. Obviously it is desirable to avoid stabilizing a deflocculated clay suspension with such thickening agents since preservatives are costly.
High solids deflocculated suspensions of calcined clay pigments have particle size distributions similar to those of hydrated filler clays. Such suspensions tend to form hard sediments during storage. Furthermore, calcined clay pigments have unusual rheological properties. The production of stable high solids suspensions is more difficult to achieve than when a typical hydrated clay is involved. In fact, calcined kaolin clays usually cannot be prepared into suspensions containing more than 60% solids by conventional techniques without producing systems which are highly dilatant. These dilatant systems resemble quicksand. When a ruler is dropped into a fluid concentrated slurry of calcined clay, it may be impossible to remove the ruler unless the ruler is removed very slowly. The shearing force applied to the suspension results in the conversion of the originally fluid system into a mass which becomes increasingly viscous at the rate of shear increases. Processing equipment such as mixers and pumps would be damaged by such highly dilatant suspensions or the equipment would stop operating.
The unusual dilatancy of concentrated aqueous suspensions of calcined kaolins is not the only difficulty to be resolved in the production of slurries suitable for shipment, handling and storage. Calcined kaolin pigments exhibit the undesirable settling characteristics of coarse hydrated filler clays. In fact, sediments of calcined clay which form during storage frequently tend to be even more difficult to break up.